A casing is provided in a well bore for a number of reasons, including isolating different zones in the drilled formation from one another, providing a strong upper foundation for the drilling fluid, and providing a smooth internal bore for installing production equipment. The casing comprises a number of steel pipes known as strings that are connected together as they are run into the bore. Each casing string will have a subsequently smaller diameter than the casing string above it.
A casing string that does not extend to the top of the well bore, but instead is anchored or suspended from inside the bottom of the previous casing string, is known as a liner. The liner therefore has a top edge which defines a step change in the internal diameter within the well bore. The use of liners can result in a substantial saving in material costs.
During the cementing process, cement slurry is first pumped down the internal bore of the casing and then displaced using another fluid, typically mud, from the lower end of the casing and up into the annular space between the casing and the rock formation. Nearer the surface, the annular spacing will be between the casing and a larger casing that was previously cemented in place.
Some of the cement slurry will adhere to the internal wall of the casing. In particular, when liners are used, slurry will accumulate at the step change in internal diameter at the top of the liner.
It is known to use scraper tools to clean the inside of a casing and remove the cement from the inside surface of the casing. The particles of cement and other debris which arise from the scraping operation can then be removed by the circulation of well fluid such as drilling mud or brine through the well.
Typically, the scraper tool comprises a cylindrical body and a number of scraping blades extending from the cylindrical body. A hypothetical “scraping diameter”, defined by the extending blades, is designed to be substantially equal to the internal diameter of the casing string to be scraped so that the scraping blades contact the casing to allow cleaning of the internal surface of the casing string.
However, known scraping tools do not provide an adequate scraping action at the top of the liner where significant amounts of slurry will accumulate. Also, contact with the top of the liner on the side of the blades can cause damage to, or bending of, the blades which impairs performance. Known tools also suffer from the disadvantage that further debris can be dislodged during extraction of the tool from the well hole, so that debris remains in the well hole after the cleaning operation.